Due to advances in device scaling for semiconductors, semiconductor device features have become smaller at the same time their aspect ratios have become larger. Consequently, semiconductor wafers have become susceptible to damage from wet cleaning and drying. This susceptibility is exacerbated by the use of new materials in the process flows for semiconductor manufacturing.
To address this susceptibility and other problems in the technology for wet cleaning and drying, a system has been developed that uses mechanical and chemical cleaning to selectively remove residue from a semiconductor wafer without damage to the wafer's device structures. This system transports a single semiconductor wafer linearly between an opposing pair of proximity heads that deliver a cleaning fluid to the wafer in an exposure time on the order of a few seconds.
In particular implementations, the cleaning fluid delivered by the opposing proximity heads is a high viscosity, non-Newtonian fluid in the form of a foam generated by mechanically mixing (a) a gas such as nitrogen (N2) and (b) a fluid containing water and a surfactant. See e.g., U.S. Published Patent Application No. 2006/0128600, entitled “Cleaning Compound and Method and System for Using the Cleaning Compound,” filed on Feb. 3, 2006, and U.S. application Ser. No. 12/185,780, entitled “Generator for Foam to Clean Substrate,” filed on Aug. 4, 2008. The disclosures of these applications are hereby incorporated by reference.
Creating useful metrics for such a foam is difficult. Chemical metrics such as the relative percentages of gas and fluid in the foam are not particularly good (e.g., relevant and/or reliable) measures of how effective the foam is at cleaning particles from the device structures on a semiconductor wafer without damage to the device structures. Consequently, a need exists for a means to statistically characterize the morphology of a foam for cleaning semi-conductor device structures. The embodiments described below include functionality for such statistical characterization, along with additional functionality which is widely applicable to this and other fields.